Method of preparing platinum coated molybdenum grids



1952 E. G. DORGELO ETAL 2,616,777

METHOD OF PREPARING PLATINUM COATED MOLYBDENUM GRIDS Filed Oct. 10, 1951 INVENTORS Eduard Gerurdus Dorgelo Johannes Wilhelmus Antonius Krol Hubertus Antho ius VunMeurs B, Ag n'r Patented Nov. 4, 1 952 METHOD OF PREPARING PLATINUM COATED MOLYBDENUM GRIDS V Eduard Gerardus Dorgclo, Johannes Wilhelmus Antonius Krol and Hubertus' Anthonius van Meurs, Eindhoven, Netherlands, as'signors to Hartford National Bank and Trust Company,

Hartford, ponm, as trustee Application October 10, 1951,.sini. 1%.250544 In the Netherlands October 17, 1950 4 Claims.

This invention relates to a method of preparing a platinum coated molybdenum grid, for an electron discharge tube having a carbonised tungsten-thorium incandescent cathode and to an electron discharge tube having a grid prepared by this method.

Primary emission can be suppressed in electron discharge tubes having a tungsten-thorium oathode by employing a molybdenum grid coated with platinum. Suppression of the primary grid-emission can be further improved by providing a car-. bon suspension on the grid coated with platinum and subsequently annealing the grid in an atmosphere of hydrogen. If the molybdenum wire is carbonised prior to being coated with platinum, the secondary emission is also reduced.

The two last-mentioned methods are relatively expensive and complicated and the secondary emission is reduced to a very low value. In transmitting tubes, however, it is often required that the secondary emission not be unduly low in order to avoid negative grid-current or grid-current characteristic curves having a negative slope. It is therefore desirable that the secondary emission be adjustable in the tube which is completely or substantially finished.

In a method of preparing molybdenum grids, coated with platinum, for electron discharge tubes containing a carbonised tungsten-thorium incandescent cathode, according to the invention subsequent to degassing the tube and subsequent to carbonising the incandescent cathode, the latter is heated to such an extent that a small amount of carbon or carbon compound vapori-ses from the cathode which deposited on the grid. The secondary emission of the grid then assumes a low value which may be adjusted within given limits to any desired value by heating the grid for a period of time.

The invention will now be explained more fully by reference to the drawing which shows typical grid-current characteristic curves of a tube employing a grid according to the invention and to the following example.

A molybdenum wire is coated by electrolytic 'means with platinum up to a thickness of at least 1 micron at a current density such as to obtain a black layer. The wire is subsequently annealed in an atmosphere of hydrogen at a temperature of about 900 C. until its colour is gray. Subsequently, a grid wound from the Wire is mounted in a tube containing a tungsten-thorium cathode. The cathode is subsequently carbonised in known manner and then heated to a very high temperature (to nearly the melting point of tungsten),

2 The primary emission and also the secondary emission of the grid is in this case small. In order to reduce the grid current, the secondary emission may be increased. by subsequently heating the grid to a very high temperature (above 1000 C.) for several minutes. Curve l of the accompanying drawing shows how the rid current is dependent upon the grid voltage V5 with a wire which is not subsequently heated and curves 2, 3 and 4 show how the grid current is dependent upon the voltage with a wire which is subsequently heated as a function of the duration of heating.

While the invention has thus been described in connection with a specific illustration, other modifications therein will be apparent to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

What we claim is:

1. A method of preparing a platinum coated molybdenum grid for use in an electron discharge tube having a carbonised tungsten-thorium cathode comprising the steps of mounting the platinum coated grid in the tube provided with a tungsten-thorium cathode, degassing the tube, carbonising the cathode, and vaporizing a small quantity of carbon from said cathode for deposit on said platinum coated grid.

2. A method of preparing a platinum coated molybdenum grid for use in an electron discharge tube having a carbonised tungsten-thorium cathode comprising the steps of electrolytically plating a layer of black platinum at least 1 micron in thickness on a molybdenum wire, annealing the platinum plated wire at a temperature of about 900 C. in hydrogen until the wire has a gray color, Winding a grid from the wire, mounting the platinum coated grid in the tube provided with a tungsten thorium cathode, degassing the tube, carbonising the cathode, and vaporizing a small quantity of carbon from said cathode for deposit on said platinum coated grid.

3. A method of preparing a platinum coated molybdenum grid for use in an electron discharge tube having a carbonised tungsten-thorium cathode comprising the steps of electrolytically plating a layer of black platinum at least 1 micron in thickness on a molybdenum wire, annealing the platinum plated wire at a temperature of about 900 C. in hydrogen until the wire has a gray color, winding a grid from the Wire, mounting desired value.

4. A method of preparing a platinum coated molybdenum grid for use in an electron discharge tube having a carbonized tungsten-thorium cathode comprising'the steps of electrolytically plating a layer of black platinum at least 1 micron in thickness on a molybdenum wire, annealing the platinum plated wire at a temperature of about 900 C. in hydrogen until the wire has a gray color, winding a grid from the wire, mounting the platinum coated grid in the tube provided with a.

tungsten-thorium cathode, degassing the tube, 15 Number carbonizing the cathode, heating the cathode to a temperature near the melting point of tungsten to vaporize a small quantity of carbon from said cathode onto said platinum coated grid, and heating said grid to a'very high temperature for several minutes to adjust the secondary emission thereof to :a desired value.

EDUARD GERARDUS DORGELO.

J OHANNES WILHELMUS ANTONIUS KROL. HUBERTUS ANTHONIUS VAN MEURS.

REFERENCES CITED The following references are of record in the file of this patent:

STATES PATENTS Name Date 1,862,138 Elsey June 7, 1932 

